1. Field of the Invention
The present invention relates to an electrical energy store having a bidirectional DC/DC converter, to a method for controlling such an energy store, and to the integration of such an energy store in an installation for storing electrical energy.
2. The Prior Art
A main problem of alternative energy such as, e.g., solar energy or wind power, is the availability thereof which principally cannot be influenced. When generating alternative energy it is therefore desirable to temporarily store energy in times of excess energy so as to be able to deliver the energy again in times of high demand and low availability of alternative energy. As storage for electrical energy, energy stores such as, e.g., batteries are well suited. Particularly suitable are redox flow batteries, which are known per se, such as, e.g., vanadium redox flow batteries. Large battery systems usually consist of a multiplicity of small cells connected so as to form cell stacks which are electrically interconnected so as to obtain a desired high power output. A redox flow battery consists, e.g., of a plurality of modules, wherein each module can comprise a multiplicity of cells or cell stacks.
It is well known that such batteries can be connected to an electrical supply grid via a DC/DC converter and a DC/AC converter, and possibly via a transformer. Thus, the battery serves for absorbing and storing excessive alternative energy and to deliver it again to the grid if the alternative energy cannot cover the demand of the grid. In this connection it is known, e.g., that the different energy sources (wind power, solar energy, battery, etc.) are connected in parallel to the grid.
By parallelizing large cell stacks or large modules from a plurality of cell stacks, e.g., in the 100 kW range, storage systems with a power capacity in the MW range can be implemented in a simple manner. For optimizing the efficiency, individual units of such units are often connected and disconnected according to demand. In this manner, self-discharge in said cells can be minimized and the energy expenditure for circulating the electrolyte can be reduced. If a storage system is built from a few cell stacks or modules, e.g., in the 100 kW range, this adaptation can take place only in very large discrete steps so that the optimum efficiency increase cannot be achieved. Moreover, damage to a unit results in a considerably reduced maximum power capacity of the storage system. When using large batteries, there is the additional disadvantage that much effort is necessary for a stable closed-loop control because the systems are controlled in master-slave operation. As soon as the number becomes high, the individual modules often can only be controlled in blocks so that often no optimal operation is ensured.
Like the setup from large parallelized units, the setup from small parallelized units, e.g., in the kW range, can also be implemented. The advantages here are very good adaptability to the power demand in terms of design and operational control, and very low influence on power provision in the event of failure of a unit. In addition, self-discharge in the case of small cell stacks or units is lower. The disadvantage here is that closed-loop control of such a system, e.g., through a central control and a bus system is complex and interference-prone.
From U.S. Pat. No. 6,184,593 B1 it is known to connect a plurality of energy sources such as, e.g., generators, turbines, batteries, solar cells, fuel cells, etc. in parallel to a DC bus via in each case one DC/DC converter, rectifier or power inverter. A load can then be supplied via a DC/AC converter on the DC bus. Here too, there is the problem of high granularity of the power range, or of the need of a complicated closed-loop control.
The present invention relates to an electrical energy store from a plurality of cell stacks, and to a method for closed-loop control of such an energy store, and to integrating such an energy store into an installation for generating electrical energy with a plurality of different energy sources. The underlying object is in particular to keep simple the closed-loop control of the electrical store consisting of a multiplicity of cell stacks.